A method of this kind is the subject of U.S. Pat. No. 3,912,927 of Hoffman corresponding to West German Auslegeschrift No. 23 32 245, the disclosures of which is hereby incorporated by reference. The apparatus employed therein only permits the second pivotal bearing to perform a single pivoting between two defined positions. Accordingly, the scanned field is as large as the individual detector elements including their separations and this brings about an image resolution that is not always adequate.